How to find which images belong to which /dev/loop?

Question

I work a lot with imaged drives, meaning a do a dd-copy of the drive in question and then work on the image instead of the drive itself.

For most work, I use kpartx to map the drive's partitions to a device under /dev/mapper/. What I'm wondering here is if there's a way to find which of the mapping belong to which image.

Consider this:

root@vyvyan:/tmp# kpartx -a -v Image1 
add map loop1p1 (254:4): 0 10240 linear /dev/loop1 2048
add map loop1p2 (254:5): 0 10240 linear /dev/loop1 12288
add map loop1p3 (254:6): 0 52848 linear /dev/loop1 22528
root@vyvyan:/tmp# kpartx -a -v Image2
add map loop2p1 (254:7): 0 33508 linear /dev/loop2 2048
add map loop2p2 (254:8): 0 39820 linear /dev/loop2 35556

Now, let's say I forget which image went to which mapping. Is there a way to let kpartx - or the kernel, or anything else - tell me which image goes where?

EDIT Also, if I accidentally rm the image-file while kpartx has added the mappings, how do you remove the mappings? kpartx wants the actual image to be present.

Answer 1

losetup (the command normally used to set them up) will tell you:

$ /sbin/losetup --list
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE
/dev/loop0         0      0         0  0 /var/tmp/jigdo/debian-7.6.0-amd64-CD-1.iso

Note that with older versions you may hat to use use -a instead of --list, and this outputs in a different and now deprecated format.

The information comes from /sys:

$ cat /sys/class/block/loop0/loop/backing_file 
/var/tmp/jigdo/debian-7.6.0-amd64-CD-1.iso

Another, possibly more portable, option is to get it from udisks:

$ udisksctl info -b /dev/loop0
/org/freedesktop/UDisks2/block_devices/loop0:
⋮
  org.freedesktop.UDisks2.Loop:
    Autoclear:          false
    BackingFile:        /var/tmp/jigdo/debian-7.6.0-amd64-CD-1.iso
    SetupByUID:         1000
⋮

losetup will also happily remove them for you, using the -d option. That just requires the loop device as a parameter; it doesn't care about the backing file/device.

Answer 2

I would also add the following. You can probe where the devices are mapped with kpartx. For example:

%> sudo kpartx -lv /dev/loop0
loop0p1 : 0 122880 /dev/loop0 8192
loop0p2 : 0 8257536 /dev/loop0 131072

This is helpful when you forget where the files are mapped. Of course the loop0p1 and loop0p2 are mapped under the directory /dev/mapper.